In multimedia broadcasting for mobile application, there is a demand for low-bit-rate transmission. For an audio signal such as that of a sound, an encoding is employed in which only a perceivable sound, for example, is encoded and transmitted taking a human auditory characteristic into consideration.
As a conventional technique for encoding, the following technique is known (for example, Japanese Patent Laid-Open No. 9-321628). An audio encoding apparatus includes: an input data memory for temporarily storing input audio signal data that is split into a plurality of frames; a frequency division filter bank for producing frequency-divided data for each frame; a psycho-acoustic analysis unit for receiving i number of frames with a frame which is sandwiched between the i number of frames, and for which a quantization step size is to be calculated, and calculating the quantization step size by using the result of a spectrum analysis for a pertinent frame and a human auditory characteristic including an effect of masking; a quantizer for quantizing an output of the frequency division filter bank with the quantization step size indicated by the psycho-acoustic analysis unit; and a multiplexer for multiplexing the data quantized by the quantizer. The psycho-acoustic analysis unit includes a spectrum calculator for performing a frequency analysis on a frame, a masking curve predictor for calculating a masking curve, and a quantization step size predictor for calculating the quantization step size.
Further, as another conventional technique, the following technique is known (for example, Japanese Patent Laid-Open No. 2007-271686). In the case of an audio signal such as that of music, many of the signal components (maskees) eliminated by compression are attenuated components that were maskers before. Thus, by giving reverberation to a decompressed audio signal, signal components that were maskers before but are now maskees are incorporated into a current signal to restore the audio signal of an original sound in a pseudo manner. Since a human auditory masking characteristic varies depending on frequency, the audio signal is divided into sub-band signals in a plurality of frequency bands, and reverberation of a characteristic conforming to a masking characteristic of each frequency band is given to the sub-band signal.
Moreover, the following technique is known (for example, National Publication of International Patent Application No. 2008-503793). In an encoder, an audio signal is divided into a signal portion with no echo and information on the reverberant field relating to the audio signal, and the audio signal is preferably divided with an expression using a very slight parameter such as a reverberation time and a reverberation amplitude. Then, the signal with no echo is encoded with an audio codec. In a decoder, the signal portion with no echo is restored with the audio codec.    [Patent Document 1] Japanese Laid-open Patent Publication No. 09-321628    [Patent Document 2] Japanese Laid-open Patent Publication No. 2007-271686    [Patent Document 3] Japanese National Publication of International Patent Application No. 2008-503793